The value of K w at a certain temperature is 6 . 25 × 10 - 14 . Calculate the pH of water.

K w = 6 . 25 × 10 - 14 Water dissociates as: H 2 O ⇌ H + + OH - ∴ K w = [ H + ] [ OH - ] = [ H + ] 2 ( ∵ [ H + ] = [ OH - ] ) ∴ [ H + ] = K w = 6 . 25 × 10 - 14 = 2 . 5 × 10 - 7 Hence, pH = - log [ H + ] = - log ( 2 . 5 × 10 - 7 ) = - log ( 2 . 5 ) + 7 = - 0 . 3979 + 7 = 6 . 6021

The value of K w at a certain temperature is 6 . 25 × 10 - 14 . Calculate the pH of water.

K w = 6 . 25 × 10 - 14 Water dissociates as: H 2 O ⇌ H + + OH - ∴ K w = [ H + ] [ OH - ] = [ H + ] 2 ( ∵ [ H + ] = [ OH - ] ) ∴ [ H + ] = K w = 6 . 25 × 10 - 14 = 2 . 5 × 10 - 7 Hence, pH = - log [ H + ] = - log ( 2 . 5 × 10 - 7 ) = - log ( 2 . 5 ) + 7 = - 0 . 3979 + 7 = 6 . 6021

Lemon juice has a pH = 2 . 1 . If all the acid in lemon is citric acid ( H Cit . ⇌ H + + Cit - 1 ) and K a for citric acid is 8 . 4 × 10 - 4 mole / litre , what is the concentration of citric acid in lemon juice?

The pH of lemon juice = 2 . 1 i.e., - log H + = 2 . 1 or [ H + ] = antilog ( - 2 . 1 ) = 7 . 943 × 10 - 3 HCit ⇌ H + + Cit - 1 Initial 1 At eq. 1 - x x x K a = H + Cit - 1 [ HCit ] ∴ 8 . 4 × 10 - 4 = 7 . 943 × 10 - 3 7 . 943 × 10 - 3 [ HCit ] Or HCit = 62 . 41 × 10 - 6 8 . 4 × 10 - 4 = 7 . 5 × 10 - 2 M